The present invention relates to an electrical resistance heater. More particularly, it relates to such a heater containing at least one heating element which comprises an electrically conductive core surrounded by insulation and a metal sheath and also a metal oversheath which is substantially completely impermeable and is compressed inwardly to provide metal-to-metal contacts between the oversheath and the internal elements. The invention provides such a heater which can be arranged to operate for long times at high temperatures in highly corrosive atmospheres. For example, such a heater can be arranged to heat a 1000 ft. long section of a well borehole at temperatures exceeding about 600.degree. C. for a number of years.